1. Field of the Invention
The present invention relates to a gas fuel supply mechanism for supplying a gas fuel to a gas combustion engine
2. Description of the Prior Art
In recent years, compressed natural gas (CNG) has widely been used as a fuel for automobiles in an effort to meet requirements for energy conservation and environmental protection. In a typical application, a natural gas is filled in a gas container under a high pressure of about 200 kg/cm.sup.2 for example, and the gas container is carried on a rear portion of an automobile for supplying the gas to the engine.
Since it is necessary to reduce the pressure of the gas to a relatively low pressure and mix the gas with air before it is introduced into the engine, it is general practice to position a pressure regulator for lowering the pressure of the gas in a gas supply passage which interconnects the gas container and the engine. For example, as disclosed in Japanese laid-open patent publication No. 62-139720, a primary pressure regulator is disposed in the vicinity of the gas container which is located in a rear portion of an automobile, and a secondary pressure regulator is disposed in the vicinity of the engine which is located in a front portion of the automobile. The pressure of the gas supplied from the gas container is lowered by the primary pressure regulator before the gas is supplied to the engine through an intermediate cabin of the automobile.
Because the primary pressure regulator usually has a high pressure reduction ratio, it is rapidly cooled and may possibly be frozen by adiabatic expansion. It is therefore necessary for the gas supply mechanism to have an independent freeze prevention mechanism such as a heater or a mechanism for introducing heat from an exhaust pipe or the like.
However, the freeze prevention mechanism is relatively heavy and expensive, and the mechanism for introducing heat from an exhaust pipe or the like is complex in structure and has poor thermal efficiency.
Japanese laid-open patent publication No. 62-170759 discloses a gas supply mechanism which includes a gas container located in a rear portion of an automobile and a gas fuel pipe extending from the gas container to an engine disposed in a front portion of the automobile for delivering a gas fuel from the gas container to the engine.
In the event of a fault of the gas supply pipe, however, the supply of the gas through the gas supply pipe has to be controlled in a suitable manner.
A device for alarming the user about a shortage of remaining gas fuel in a gas container is disclosed in Japanese laid-open patent publication No. 63-222235, for example. The disclosed device has a warning indicator which can be actuated when a certain pressure difference is detected by a differential pressure sensor between the pressures in first and second pressure chambers that are positioned between a gas container and a pressure regulator.
The warning indicator suffers a large error because the operation of the warning indicator depends only upon the pressure of the gas fuel, without concern over other conditions such as the temperature. The amount of remaining gas fuel in the gas container should be indicated as accurately as possible because any substantial fuel indication errors would lead to a failure of the automobile to travel a desired distance.
If an equation of state of an ideal gas, i.e., G=P.multidot.V/R.multidot.T (G: the mass, V: the volume, P: the pressure, T: the temperature, and R: a gas constant) were used to determine an amount of remaining gas fuel, then a high-pressure gas fuel such as CNG would suffer a large deviation (e.g., the weight of a fuel which is 100% methane is 1.24 times the weight of an ideal gas under P=200 kg/cm.sup.2 at T=20.degree. C.). It is therefore necessary to use a real gas conversion coefficient (CF) in the determination of an amount of remaining gas fuel.
If a gas fuel is 100% ethane, then the real gas conversion coefficient is 1.34. As different gases have different real gas conversion coefficients, the properties of gases have also to be taken into account.
As with gasoline engines, gas combustion engines may incorporate a fuel injection system for the purposes of purifying exhaust gases, improving the rate of fuel consumption, increasing the output power, and improving the ease with which the engine can start. The fuel injection system controls the mixture ratio of air and fuel and inject the mixture at the controlled mixture ratio into engine cylinders.
Specifically, a gas supplied from the gas container under a high pressure of 200 kg/cm.sup.2 for example, is depressurized to a preset pressure by a pressure regulator, and then mixed with air. The mixture is thereafter introduced into engine chambers, and then injected into engine cylinders from injectors.
The gas depressurized by the pressure regulator has a problem in that unless the preset pressure is controlled precisely, the air-fuel ratio is varied failing to achieve desired characteristics, and the mixture has poor responses. The preset pressure may be varied due to a change in the gas pressure when the automobile is accelerated or decelerated, a degradation of pressure control characteristics caused by the hardening of rubber of the pressure regulator at low temperatures, and the different characteristics of individual pressure control valves. In the case where fuel injection valves are electrically actuated, they are subject to a delay in their response before they are actually operated after drive voltages have been applied thereto. The response delay is responsible for the fuel injection valves to fail to be opened to a required degree.
Therefore, the presently available gas fuel supply mechanisms for gas combustion engines have much to be improved. There has been a demand for a gas fuel supply mechanism which will solve the above various problems.